Seriously though, when is the last time you changed your gear oil? Mine was difficult getting into 1st and 3rd and after changing the oil, shifting is much easier. Beyond that, it could be your shifter seat not allowing you to fully shift in that direction.

Synchros could be shot. I had a Mitsubishi truck that lost 3rd and afterdraining the fluid and seeing too many metal flakes and then taking off thecover allowed me to see a synchro that had broken apart. However, in your caseit may not be a synchro but a shift fork, can also be a shifter ball that has disintegratedcausing too much slack, which can cause engagement issues.

Drain the oil into an empty pan and look for foreign material like brassflakes and/or pieces.

Common result of a shift rail issue, or detent ball. However extremely dirty trans fluid can cause issues like this as well. Is it just really difficult to get it in those gears, resulting in you having to force it, or will it just not go in those gears at all, any time? (driving, sitting still, engine on or off, etc)